In vertebrate species it consists of two main parts, the central nervous system CNS and the peripheral nervous system PNS. The CNS contains the brain and spinal cord. The PNS consists mainly of nerves, which are enclosed bundles of the long fibers or axons, that connect the CNS to every other part of the body.

Nerves that transmit signals from the brain are called motor or efferent nerves, while those nerves that transmit information from the body to the CNS are called sensory or afferent. Most nerves serve both functions and are called mixed nerves.

The PNS is divided into a somatic and b autonomic nervous system, and c the enteric nervous system. Somatic nerves mediate voluntary movement. The autonomic nervous system is further subdivided into the sympathetic and the parasympathetic nervous systems. The sympathetic nervous system is activated in cases of emergencies to mobilize energy, while the parasympathetic nervous system is activated when organisms are in a relaxed state.

The enteric nervous system functions to control the gastrointestinal system. Both autonomic and enteric nervous systems function involuntarily. Nerves that exit from the cranium are called cranial nerves while those exiting from the spinal cord are called spinal nerves. The nervous system derives its name from nerves, which are cylindrical bundles of fibers the axons of neuronsthat emanate from the brain and spinal cord, and branch repeatedly to innervate every part of the body. Nerves are large enough to have been recognized by the ancient Egyptians, Greeks, and Romans, but their internal structure was not understood until it became possible to examine them using a microscope.

Equally surprising is the fact that the concept of chemical transmission in the brain was not known until around Henry Hallett Dale and Otto Loewi.

We began to understand the basic electrical phenomenon that neurons use in order to communicate among themselves, the action potential, in the decade of Alan Lloyd Hodgkin, Andrew Huxley and John Eccles. It was in the decade of that we became aware of how basic neuronal networks code stimuli and thus basic concepts are possible David H.

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Hubel, and Torsten Wiesel. The molecular revolution swept across US universities in the decade of It was in the decade of that molecular mechanisms of behavioral phenomena became widely known Eric Richard Kandel. The neurons that give rise to nerves do not lie entirely within the nerves themselves—their cell bodies reside within the brain, spinal cord, or peripheral ganglia Glial cells named from the Greek for "glue" are non-neuronal cells that provide support and nutrition, maintain homeostasis, form myelin, and participate in signal transmission in the nervous system.

In the human brain, it is estimated that the total number of glia roughly equals the number of neurons, although the proportions vary in different brain areas. In higher animals striated muscles occur in bundles attached to bone to provide movement and are often arranged in antagonistic sets. Smooth muscle is found in the walls of the uterus, bladder, intestines, stomach, oesophagus, respiratory airways, and blood vessels.

Cardiac muscle is found only in the heart, allowing it to contract and pump blood round the body. Sharks and rays are basal fish with numerous primitive anatomical features similar to those of ancient fish, including skeletons composed of cartilage.

Their bodies tend to be dorso-ventrally flattened, they usually have five pairs of gill slits and a large mouth set on the underside of the head. The dermis is covered with separate dermal placoid scales. They have a cloaca into which the urinary and genital passages open, but not a swim bladder. Cartilaginous fish produce a small number of large, yolky eggs. Some species are ovoviviparous and the young develop internally but others are oviparous and the larvae develop externally in egg cases.

The bony fish lineage shows more derived anatomical traits, often with major evolutionary changes from the features of ancient fish. They have a bony skeleton, are generally laterally flattened, have five pairs of gills protected by an operculum, and a mouth at or near the tip of the snout. The dermis is covered with overlapping scales. Bony fish have a swim bladder which helps them maintain a constant depth in the water column, but not a cloaca. They mostly spawn a large number of small eggs with little yolk which they broadcast into the water column Mammals are a diverse class of animals, mostly terrestrial but some are aquatic and others have evolved flapping or gliding flight.

They mostly have four limbs but some aquatic mammals have no limbs or limbs modified into fins and the forelimbs of bats are modified into wings. The legs of most mammals are situated below the trunk, which is held well clear of the ground.

The bones of mammals are well ossified and their teeth, which are usually differentiated, are coated in a layer of prismatic enamel.

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The teeth are shed once milk teeth during the animal's lifetime or not at all, as is the case in cetaceans. Mammals have three bones in the middle ear and a cochlea in the inner ear. They are clothed in hair and their skin contains glands which secrete sweat. Some of these glands are specialized as mammary glands, producing milk to feed the young. Mammals breathe with lungs and have a muscular diaphragm separating the thorax from the abdomen which helps them draw air into the lungs.

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